1. Field of the Invention
The present invention relates to an adaptive hybrid circuit for minimizing crosstalk from a transmitter to a receiver in the same station when the transmitter and receiver are coupled to a two-wire transmission line or communication channel, such as a coaxial or twisted pair cable, of a local area network comprised of a plurality of transmitter/receiver stations coupled to the same two-wire communication channel.
2. Description of the Prior Art
In the transmitter/receiver circuitry of a bus-type local area network based on CSMA/CD (carrier sense multiple access with collision detection), a hybrid circuit can be used for the separation or isolation of simultaneously transmitted and received signals at one of the stations in the network. This separation allows the implementation of collision detection at larger transmit to receive level ratios than without a hybrid. Thus, larger transmission distances can be achieved by using a hybrid. However, a hybrid must be designed for a fixed nominal transmission line impedance. This means that the maximum attenuation of the transmitted signal crosstalk into the received signal is obtained for only one transmission line impedance. In addition, the crosstalk attenuation for a given transmission line impedance will depend on the tolerances of the components that comprise the hybrid. Thus, any change in the impedance of the transmission line or in the values of the components in the hybrid, such as due to a temperature change, can prevent the hybrid from optimally cancelling the transmitted signal crosstalk from the received signal.
Adaptive hybrid circuits have been proposed in U.S. Pat. No. 3,810,182 and in British patent specification No. 1,492,045 for cancelling the crosstalk from a transmitter to a receiver at the same station.
U.S. Pat. No. 3,810,182 has an adaptive hybrid circuit interposed between the transmitter and the receiver at one end of the transmission channel and correlates signals in the transmission channel with signals from the transmitter to adjust both the gain and phase shift of the signal from the transmitter until the signal component in the transmission channel from the transmitter is filtered from the transmission channel signal so that only the receiver signal component is passed into the receiver.
British patent specification No. 1,492,045 discloses an audio frequency adaptive hybrid circuit for use in telephone circuits wherein a 4 wire transmit port, a 4 wire receive port and a 2 wire channel port are isolated from one another by transformers. An impedance synthesis network, comprising a tapped delay line network, and a correlation control loop for controlling adjustable tap weights in the tapped delay line network are utilized to cancel the crosstalk.